Working at night and sleeping during the day is probably the most common cause of circadian rhythm disruption in modern society. Several studies indicate that night shift work becomes increasingly burdensome as a worker grows older. As the work force ages, the number of older workers who will be asked to adapt to unconventional schedules will increase. thus there is a pressing need to develop scientifically-based strategies to assist the older worker in adapting to the rigors of a night-active schedule. In this project, we will test the hypotheses that the endogenous circadian pacemaker becomes less flexible (more difficult to "reset") as one grows older, and as a result, circadian desynchrony is more pronounced in the aging night worker. Circadian desynchrony is operationally defined as the degree of incongruence between an individual's self-selected sleep/activity cycle and his/her underlying physiological rhythms. After developing new methods for assessing circadian desynchrony, we will conduct clinical trials with melatonin administration aimed at reducing it. Specifically, we will compare the degree of circadian desynchrony in young, middle-aged, and older night shift workers who are working permanent and regularly rotating shifts. Dependent variables measured will include 1) The phase of the primary circadian pacemaker in the suprachiasmatic nucleus (SCN), as determined by the timing of melatonin production, measured in either plasma and/or saliva samples. 2) The phase and amplitude of the sleep propensity rhythm (SPR), determined by an ultrashort sleep-wake schedule. 3) Actual sleep times chosen by the subjects (as well s their daytime activity patterns), as determined by wrist actigraphy. 4) Sleep quality and quantity, as determined by periodic "in home" portable polysomnography. 5) Illumination history, as determined by continuous photometric monitoring. 6) Psychomotor performance, as determined by a well-validated, simple reaction-time test. 7) Subjective dysphoria, as determined by a visual analogue scale of vigor and affect. From these measures, both the degree of circadian desynchrony, as well as its impact on sleep, performance and subjective well-being will be assessed comparing workers through the life span. In an attempt to correct circadian desynchrony in night workers, melatonin administration will be tested as a method to align underlying circadian rhythms to the sleep times required by the night work schedule.